This invention relates to the art of slitting a web of sheet steel into a plurality of strips and winding them onto a rotatable take-up drum and, more particularly, to a method for eliminating undue slack in the strips as they are wound into uniformly tight coils on the drum.
The problem solved by the present invention is caused by the fact that when rolled sheet steel is formed in a rolling mill the sheets are not perfectly flat but are slightly convex so that the center of the sheet is thicker than the edges. This causes problems in conventional slitting operations where the sheets in the form of rolls are unwound and slit or cut into a number of adjacent strips which are then wound into coils on a take-up or recoiling drum. One such drum which has a selectively expansible and contractable outer surface for easy removal of the coils is the subject of U.S. Pat. No. 3,815,839, which issued June 11, 1974 to the owner of the instant invention and has the same inventor. The subject matter of that patent is hereby incorporated by reference as though it were fully set forth herein.
The problem caused by the varying thicknesses of the strips is that since all of the strands are rolled onto a single drum the coils at the outer ends tend to be looser than the ones in the middle where the strips are thicker. Loose coils are harder to band together and palletize and are dangerous to handle because if they are dropped they can uncoil violently like a giant spring.
These problems could possibly be solved by winding each strip on its own take-up drum so that the rotational velocity of the drums could be individually adjusted to maintain the tension in the strips within an acceptable range. This alternative, however, would be prohibitively expensive and impractical for commercial operations.
Attempts to solve the problem have been made by providing a tensioning mechanism between the slitting mechanism and the take-up drum so that a uniform tension can be maintained in the strips as they are rolled onto the drum. Although this solution eliminates the loose coil problem, slack is created in the portion of the outer strands upstream from the tensioning mechanism, which can result in loops being formed in the outermost strips extending ten feet or more below the normal line of travel. It has been found that a pit or large vertical clearance is necessary to accommodate the loops.
Where a tensioning mechanism and accompanying pit or vertical clearance are impractical, the prior art teaches that a paper stuffer can be effective in eliminating the slack. The paper stuffer is a machine which interleaves a strand of paper in selected portions of the coil as it is being wound around the take-up drum to compensate for the smaller thickness of the outermost strips and provide a tightly wound coil.
Each of these alternatives, although workable, has proved to be economically detrimental to full realization of the commercial potential of a slitting operation. Further, formation of the loops when slack is created and the paper stuffer machine are potentially dangerous to workers. More particularly, an independently rotatable drum for each strip is obviously a prohibitively expensive alternative. The utilization of a separate tensioning mechanism and the space required for a pit or vertical clearance have been used, but they add noticeably to the expense of the slitting operation. Loops in the steel strips which extend ten feet or more below the normal line of travel are subject to unpredictable movement, which increases the danger of working near the slitting equipment. The paper stuffer is another piece of expensive equipment and must be located in close proximity to the takeup drum so that workers who are servicing or otherwise working near the stuffer expose themselves to the hazards of the moving steel strips.